1. Field of the Invention:
The present invention relates to crystallized polyamides, and, more especially, to crystallized polyamides produced from terephthalic acid, 2-methylpentamethylenediamine and a hindered aromatic diamine.
2. Description of the Prior Art:
Polyamides produced from aliphatic diamines and from aliphatic or aromatic dicarboxylic acids have long been known to this art. Depending on their composition, these polyamides are crystallized materials (and thus are designated "semicrystalline" polymers) having high melting points (Tm) or completely amorphous polymers having relatively low glass transition temperatures (Tg).
Semicrystalline polyamides are advantageously used for the production of shaped articles which have good thermomechanical strength at elevated temperatures. The most common semicrystalline polyamide is polyamide 66 (polymer of hexamethylenediamine and adipic acid); having a Tm on the order of 265.degree. C., this polyamide can be easily melt-processed but, because of a low Tg of approximately 50.degree. C., its fields of application are limited to those involving exposure to temperatures which remain below 100.degree. C. Other semicrystalline polyamides are known to the art which have a higher Tg and consequently a better retention of mechanical properties as a function of temperature. Typical polyamides of this type are, for example, those produced from terephthalic acid, isophthalic acid (20 to 40 mol % in the mixture of diacids) and hexamethylenediamine (see particularly U.S. Pat. Nos. 3,941,755 and 4,607,073), which have a Tg of from approximately 120.degree. to 140.degree. C. The thermal stability of such polyamides is apparent, but conversion of these copolyamides requires very high processing temperatures because of their high melting points, which range from approximately 320.degree. to 340.degree. C. The possibility of obtaining Tg values above 100.degree. C., while at the same time having Tm values which preferably remain below 290.degree. C., such that the polymer can be processed according to the usual techniques employed for converting polyamide 66, exists and is present in the polyamide produced from terephthalic acid and 2-methyl-1,5-pentamethylenediamine (JA-A-69/019,551) whose Tg is equal to 142.degree. C., whereas the Tm is equal to 285.degree. C.
Since the Tg is an important factor that will determine the thermal stability of the polymers, serious need continues to exist in this art for polyamides having a Tg above 142.degree. C. and a Tm which remains below 290.degree. C.